1. Field of the Invention
The present invention relates to a semiconductor optical element that can enhance power and improve operation reliability by using C or Mg as a p-type impurity.
2. Background Art
Heretofore, Zn has been widely used as a p-type dopant for a semiconductor optical element (refer for example to Japanese Patent Laid-Open No. 11-054828 Summary of the Invention). Table 1 shows the impurity to be added, carrier concentration, and film thickness of each layer in a conventional semiconductor optical element. Here, as an example, a laminated structure of a semiconductor optical element used in a light source that uses a GaAsP/InGaP-based material used for exciting a solid laser in an 808-nm band.
TABLE 1CarrierconcentrationFilm thicknessLayer nameImpurity(1018/cm3)(nm)p-type GaAs contactZn10.0-30.0100-500 layerp-type InGaP-BDRZn1.0-3.020-100layerp-type AlGaInp cladZn1.0-2.0500-1500layerInGaAsP guide layer——500-1500GaAsP active layer——5-12InGaAsP guide layer——500-1500n-type AlGaInp cladSi0.5-1.5500-1500layern-type GaAs bufferSi0.5-1.5200-700 layern-type GaAs substrateSi0.7-1.0—
In the constitution of Table 1, an n-type GaAs (or AlGaAs) buffer layer to which Si is added as an n-type impurity, an n-type AlGaInP clad layer to which Si is added, a InGaAsP guide layer without adding impurities, a GaAsP active layer without adding impurities, an InGaAsP guide layer without adding impurities, a p-type AlGaInP clad layer to which Zn is added as a p-type impurity, a p-type InGaP-BDR (band discontinuity reduction) layer to which Zn is added, and a p-type GaAs contact layer to which Zn is added are sequentially laminated on an n-type GaAs substrate.
However, Zn used as a p-type impurity is easily diffused by heat treatment during growing or semiconductor-laser fabricating. Therefore, when the overflow of implanted carriers (electrons in this case) is to be suppressed by raising the Zn concentration of the p-type AlGaInP clad layer, the diffusion of Zn into the GaAsP active layer, in which impurities cannot be mixed under normal conditions is occasionally observed.
Particularly in conventional structures, if a high concentration (for example, 1019/cm3 or more) of Zn is added to a p-type GaAs contact layer, the p-type GaAs contact layer acts as a supply source of Zn, and the diffusion of Zn into the GaAsP active layer is further accelerated.
If an impurity is mixed in the active layer, since a nonradiative recombination center is formed causing problems, such as the lowering of emission intensity and the movement of the PN-junction (deviation from the design value), a semiconductor optical element that has favorable characteristics cannot be manufactured. FIG. 4 shows the results of a reliability test of a conventional semiconductor optical element having the structure shown in Table 1. Sudden death is observed at about 400 hours after starting the reliability test, and the structure cannot be the to have sufficient reliability.